The invention is directed to an adaptive control system and related method. More particularly, the invention is directed to an adaptive control system with the capability to prevent or reduce undesired adaptation of a control system due to selected characteristic(s) of the plant or control system.
Adaptive control systems have the capability to adapt control response to changing conditions within the plant being controlled or the plant""s operating environment. Adaptation to changing plant or environmental conditions provides enhanced control response for many kinds of plants, and may be required for certain types of plants that cannot be controlled with static control systems. However, conventional adaptive control systems have a tendency to adapt to plant or control system characteristics to which they should not. The control response of adaptive control systems can be greatly diminished when subjected to certain characteristic(s) of the plant or system, and can be rendered unstable in attempting to adapt to these characteristics. An example of a characteristic that could adversely affect an adaptive control system is a control or authority limit imposed on the control elements of an adaptive control system. If an operator generates a command signal that exceeds the ability of the control system or plant to respond, adaptation of the control system can render the control system unstable. It is desirable to reduce or prevent the impact of such characteristic(s) from adversely affecting the adaptive control system""s performance.
The methods, apparatus, and system, of the invention overcome the disadvantages noted above.
A first method of the invention comprises generating a hedge signal to avoid adaptation to a characteristic of at least one of an adaptive control system and a plant controlled by such system. The first method can comprise modifying a commanded state signal with the hedge signal. The first method can also comprise generating a reference model state signal using the commanded state signal modified by the hedge signal. The first method also can comprise generating a tracking error signal based on the reference model state signal and a plant state signal, and generating an adaptive control signal to adapt control response of the adaptive control system. Through compensation for the characteristic in the tracking error signal, the adaptive control signal can be generated so as not to significantly adapt to the characteristic. Hence a characteristic of the plant or control system that would impair or be detrimental to control system""s performance and/or stability can be hedged out of the adaptive portion of the control system to prevent adverse impact on control of the plant. The hedge signal can be generated based on a difference between a first signal derived from a plant model not having the characteristic to be hedged, and a second signal derived from a plant model having the characteristic. The first signal can generated based on an input control signal and a plant state signal in addition to the plant model not having the characteristic. The second signal can be generated using a command control signal and a plant state signal, in addition to the plant models with the characteristic. The input control signal can be generated based on a commanded state signal, a plant state signal, and an adaptive control signal, and the command control signal can be generated using the input command signal modified by a control allocation and a control characteristic imposed by a controller. The input control signal and command control signal can be used to hedge a characteristic of the control system to which adaptation is not to be performed. The method can also include generating a display based on the input control signal. An operator can use the display to generate a command control signal. In this aspect of the invention, the operator""s control and response can be hedged.
A second method of the invention is executed by an adaptive control system. The second method comprises generating an input control signal based on a commanded state signal, a plant state signal, and an adaptive control signal. The second method also comprises generating a command control signal based on a commanded state signal, a plant state signal, an adaptive control signal, and further based on control allocation and a control characteristic of a controller used to generate the command control signal. The second method further comprises supplying the command control signal to an actuator, controlling a state of the plant based on the command control signal, sensing a state of the plant, and generating a plant state signal based on the sensing of the plant. The second method comprises generating a first signal based on the input control signal, the plant state signal, and a plant model without a plant characteristic to which the adaptive control system is not to adapt. The second method also comprises generating a second signal based on the command control signal, the plant state signal, and a plant model with the plant characteristic to which the adaptive control system is to adapt. The second method further comprises generating a hedge signal by differencing the first and second signals, and generating a reference model state signal by modifying the commanded state signal with the hedge signal to include the effect of the control allocation and control characteristic on plant state from the reference model state signal. The second method further comprises comparing the plant state signal and the reference model state signal, generating a tracking error signal based on the comparing step, and generating the adaptive control signal based on the tracking error signal. The second method can comprise generating a reference model signal based on the commanded state signal, the hedge signal, and a reference model signal derived from a reference model representing the target response of the plant, the reference model signal to generate the input control signal. The second method can also comprise generating a reference model signal based on the commanded state signal, the hedge signal and a reference model signal derived from a reference model representing the target response of the plant, the reference model signal, to generate the command control signal. The second method can also comprise generating a linear control signal based on the tracking error signal, generating a reference model signal based on the commanded state signal, the hedge signal, and a reference model, and generating a pseudo- control signal based on the linear control signal, the reference model signal, and the adaptive control signal, and the pseudo-control signal. The adaptive control signal can be generated with the plant state signal. The adaptive control signal can be generated with a neural network having connection weights adjusted based on the tracking error signal and the pseudo-control signal. The neural network maps the plant state signal to the adaptive control signal. The plant state signal can also be used to generate the adaptive control signal. The second method can comprise generating the commanded state signal based on a control action from an operator. The operator can be human, and the method can comprise generating a display based on the plant state signal. The display can be used by the operator to generate the commanded state signal. The second method can comprise generating the commanded state signal based on a signal generated by an operator that is a computer. The second method can also comprise generating a display for an operator based on the input control signal so that the operator can generate the command control signal based on the display.
An apparatus of the invention can be used in an adaptive control system for controlling a plant. The apparatus is a hedge unit coupled to receive at least one control signal and a plant state signal. The hedge unit generates a hedge signal based on the control signal, the plant state signal, and a hedge model including a first model having a characteristic to which the adaptive control system is not to adapt, and a second model not having the characteristic to which the adaptive control system is not to adapt. The hedge signal can be used in the adaptive control system to remove the characteristic from a signal supplied to an adaptation law unit of the adaptive control system so that the adaptive control system does not adapt to the characteristic in controlling the plant. The characteristic to be hedged by the hedge unit can be a time delay between generation of the commanded state signal by the controller at a time, and receipt by the controller of the plant state signal resulting from the commanded state signal generated at the time. Also, the characteristic can be a time delay between generation of a state by the plant and sensing of the state of the plant by the sensor to generate the plant state signal. Alternatively, the characteristic can pertain to a control limit of the actuator used to control the plant. The control limit can be due to actuator end points, actuator dynamics, a rate limit of the actuator, or quantization effects associated with the actuator, for example.
An adaptive control system of the invention is coupled to receive a command state signal indicative of a target state of a plant controlled by the adaptive control system. The adaptive control system comprises a controller coupled to receive the commanded state signal, a plant state signal, and an adaptive control signal. The controller generates an input command signal based on the commanded state signal, the plant state signal, the adaptive control signal, and a control model. The controller generates a command control signal based on the commanded state signal, the plant state signal, the adaptive control signal, the control model, control allocation of the controller, and at least one control characteristic of the controller. The controller is coupled to supply the command control signal to the plant to control the plant""s state. The actuator is coupled to receive the command control signal, and affects physical control of the plant""s state using the command control signal. The adaptive control system can comprise a sensor coupled to sense the plant state, that generates a plant state signal based on the sensed plant state. The adaptive control system also comprises a hedge unit coupled to receive the input control signal, the command control signal, and the plant state signal. The hedge unit generates a hedge signal to modify the command state signal based on the input control signal, the command control signal, the plant state signal, and a hedge model indicative of a characteristic of at least one of the plant and the adaptive control system, to remove the effect of the characteristic on a tracking error signal. The adaptive control system also comprises a reference model unit coupled to receive the command state signal and the hedge signal. The reference model unit generates a reference model state signal based on the commanded state signal and a hedge signal. The adaptive control unit also comprises a comparator unit coupled to receive the reference model state signal and the plant state signal. The comparator unit generates a tracking error signal based on a difference between the plant state signal and the reference model state signal. The adaptive control system also includes an adaptation law unit coupled to receive the tracking error signal. The adaptive control system generates the adaptive control signal based on the tracking error signal. The adaptation law unit is coupled to supply the adaptive control signal to the controller. The controller can generate the input control signal and the command control signal further based on the reference model state signal. The characteristic to be hedged by the adaptive control system can be time delay between generation of the commanded state signal by the controller at a particular time, and receipt by the controller of the plant state signal resulting from the commanded state signal generated at the particular time. Alternatively, the characteristic can be a time delay between generation of a state by the plant in response to the command control signal, and sensing of the state of the plant resulting from the command control signal. Further, the characteristic can be a control limit of the actuator used to control the plant. The control limit can be due to actuator end points, actuator dynamics, a rate limit of the actuator, or quantization effects of the actuator, for example. The commanded state signal can be generated by an operator, and the adaptive control system can comprise an operator interface unit coupled to receive the plant state signal. The operator interface unit relays the plant state to the operator. The command unit can be used by the operator to generate the command state signal based on the operator""s control action. The operator interface can be a display generated based on the plant state signal. The operator can be a human being that generates the control action to the command unit to generate the commanded state signal. The commanded state signal is generated by a machine operator based on the plant state signal. The adaptation law unit can comprise a neural network having connection weights determined by the tracking error signal. The neural network can map the plant state signal to the adaptive control signal based on the connection weights to generate the adaptive control signal. The controller can generate a pseudo-control signal based on the commanded state signal and the plant state signal. The controller can be coupled to supply the pseudo-control signal to the neural network to adjust the connection weights of the neural network. The controller can comprise a dynamic inversion unit to generate the command control signal.
These together with other objects and advantages, which will become subsequently apparent, reside in the details of construction and operation of the invented methods, apparatus, and article as more fully hereinafter described and claimed, reference being made to the accompanying drawings, forming a part hereof, wherein like numerals refer to like parts throughout the several views.